ABC transporters pump a variety of small molecules and peptides across cell membranes. Mutations in ABC transporter genes cause severe defects in man. Drugs that modulate ABC transporter function would have utility as anti-microbials through inhibition of essential microbial ABC transporters, as well as in the states of atherosclerosis, macular degeneration, diabetes, Parkinson's disease, and multiple drug resistance developing during cancer treatment. However this protein family has not been systematically investigated as a target for pharmaceutical design. Recent X-ray structural studies have elucidated the overall arrangement of the ABC transporter component proteins in the membrane, and also revealed critical features of the ATP binding site of the isolated nucleotide binding domains. The Aim of this proposal is to capitalize on these breakthroughs by setting up a program for the structure-based design of drugs that will modulate the activity of the transporters for medicinal benefits. The nucleotide binding domains will be cloned from 8 different human, and 2 different essential bacterial, ABC transporters, and these will be engineered for over-expression in bacteria. The proteins made will be purified and used for development of ATP binding assays and nucleotide binding domain dimerization assays. The assays will be used to screen for compounds that interact at the ATP-binding site. The success of finding active compounds will be enhanced through molecular modeling work to select a library of compounds most likely to interact with the ATP binding site, as defined by recently published three dimensional X-ray structures of ABC transporter nucleotide binding domains. Purified nucleotide binding domain proteins will also be used to initiate X-ray structural studies in-house, with the objective of creating a system for the visualization of compound binding modes through co-crystallization of nucleotide binding domains with the active compounds. By the end of Phase I a clear picture should emerge of which members of the ABC transporter family are most amenable to drug discovery efforts using rational drug design methods.